An attenuation map is usually created with combined magnetic resonance/positron emission tomography imaging (MRI-PET imaging) in a magnetic resonance/positron emission tomography system (MRI-PET system). The MRI-PET system typically includes a magnetic resonance imaging scanner (MRI) and a positron emission tomography scanner (PET). MRI-PET imaging typically includes at least one MRI imaging scan (MRI imaging) and at least one PET imaging scan (PET imaging).
The attenuation map can be determined based on magnetic resonance data (MRI data), which is preferably acquired with MRI imaging. Positron emission data (PET data) acquired with PET imaging is usually corrected by way of the attenuation map. Typically, in MRI, a maximum field of view, in which the MRI data can be acquired in principle, is limited in all three spatial directions due to physical and/or technical restrictions. Such restrictions are usually non-linearity of an MRI magnetic gradient field and/or localized homogeneity of an MRI main magnetic field, which can result in B0 field inhomogeneities.
For this reason, typically a scan region of the MRI imaging is limited to a region within the maximum field of view in which the aforementioned restrictions are preferably within specific limit values thereby usually enabling substantially distortion-free MRI imaging. During MRI imaging, distortion typically occurs outside the maximum field of view in a peripheral region of the MRI as a result of which the image quality of magnetic resonance images (MRI images) reconstructed using the MRI data can be impaired. In particular in this context, distortion means that a signal value at a predetermined location of an examination object appears at another location in the MRI images of the examination object.
MRI-PET imaging typically requires the scan region to fill the maximum field of view, wherein the maximum field of view is preferably defined as broadly as possible. This is because, if the examination object is a patient, the patient's extremities, in particular the arms, are located in a peripheral region in which the aforementioned restrictions are typically outside the specific limit values. Nevertheless, the attenuation map is typically determined for an entire body of a patient, including the arms. If the arms are located in the peripheral region the MRI data may have deviating signal values as a result. However, usually even a slight deviation in the MRI data is serious with respect to the determination of the attenuation map because there is typically an exponential dependence between attenuation values within the attenuation map and the MRI data.
DE 10 2010 006 431 A1 describes a method for determining a location of a subarea of an examination object in a magnetic resonance system, wherein the subarea is arranged at the edge of a field of view of the magnetic resonance system, wherein at least one slice position is determined for an MR image in which the B0 field at the edge of the MR image satisfies a predetermined homogeneity criterion, wherein an MR image is acquired in the specific slice position, wherein the MR image contains the subarea at the edge of the field of view and wherein the location of the subarea of the examination object is determined by the location of the subarea in the MR image.
A method disclosed in DE 10 2012 203 782 A1 describes a further possibility for avoiding the aforementioned restrictions. DE 10 2012 203 782 A1 discloses a method for performing combined magnetic resonance/positron emission tomography imaging of an examination object in a MR-PET system with which inter alia magnetic resonance data is acquired using a first readout gradient field, wherein the first readout gradient field is selected such that, at a predetermined location of the field of view of the magnetic resonance system, distortion caused by non-linearity of the first readout gradient field and distortion caused by B0 field inhomogeneity substantially cancel each other out.
Typically, such a method requires the readout gradient field to be adapted separately for two sides of the scan region, for example the sides with the two arms of the patient. Therefore, typically, there are two imaging scans wherein initially a first side of the patient is acquired and subsequently a second side of the patient is acquired. For these two MRI imaging scans, typically an amplitude of the readout gradient field is set independently for each side and one scan taken in the head-foot direction and another in the foot-head direction. Therefore, the two MRI imaging scans can require a patient bench bearing the patient to be moved to-and-fro, thereby prolonging the overall scan time.
Furthermore, sequence parameters of the two MRI imaging scans are not usually specifically adapted to the patient, in particular to the location and/or anatomy of the patient. In other words, typically the sequence parameters of the MRI imaging scans are defined independently of whether a, and/or which, patient is located on the patient bench.